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Total Reflection X-ray Fluorescence Analysis (TXRF) using the high flux SAXS camera
Nuclear Instruments and Methods in Physics Research A 482 (2002) 569–572
Letter to the Editor
Total Reflection X-ray Fluorescence Analysis (TXRF) using the high flux SAXS camera P. Wobrauscheka,*, C. Strelia, G. Pepponia, A. Bergmannb, O. Glatterc a
Atominstitut, TU Wien, Stadionallee 2, 1020 Vienna, Austria b Anton Paar GmbH, Graz, Austria c Institute of Chemistry, Graz, Austria Received 2 January 2002; accepted 22 January 2002
Abstract Combining the high photon flux from a rotating anode X-ray tube with an X-ray optical component to focus and monochromatize the X-ray beam is the most promising instrumentation for best detection limits in the modern XRF laboratory. This is realized by using the design of a high flux SAXS camera in combination with a 4 kW high brilliant rotating Cu anode X-ray tube with a graded elliptically bent multilayer and including a new designed module for excitation in total reflection geometry within the beam path. The system can be evacuated thus reducing absorption and scattering of air and removing the argon peak in the spectra. Another novelty is the use of a Peltier cooled drift detector with an energy resolution of 148 eV at 5.9 keV and 5 mm2 area. For Co detection limits of about 300 fg determined by a single element standard have been achieved. Testing a real sample NIST 1643d led to detection limits in the range of 300 ng/l for the medium Z. r 2002 Published by Elsevier Science B.V.
1. Introduction Total Reflection X-ray Fluorescence Analysis (TXRF) is a special technique in energy dispersive X-ray fluorescence analysis achieving detection limits in the pg range (or ppb-levels) with standing anode 2 kW fine focus X-ray tubes [1,2]. The technique is based on the total external reflection of X-rays on the smooth polished surface of a quartz or Si reflector if the beam impinges under an angle lower than the critical angle. TXRF requires therefore a low divergent beam with high intensity. Synchrotron radiation is the ideal source
for TXRF and allows in reducing the detection limits down to the fg level [3]. To improve the laboratory performances of TXRF an increase of photon flux for sample excitation is required. A high brilliant rotating anode tube with focusing optics should improve the detection limits. Experiments with the high flux SAXS (small angle X-ray scattering) Camera at the Institute of Chemistry, University of Graz, have been performed.
2. Experimental *Corresponding author. E-mail address: [email protected] (P. Wobrauschek).
Laboratory X-ray sources emit a highly divergent beam. The Kratky compact camera is
0168-9002/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 4 6 3 - 1
570
P. Wobrauschek et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 569–572
Elliptically bent multilayer X-ray A optics
Block collimation
Detection plane of the scattered radiation
Sample position
system
R
B2 f
H
w
DS
k 1'
X
Rotating anode generator
k 2'
k2
O1 k1
p
O2
m0
B1 C 2
C 2
C 2
3C 2
C = 180 mm Fig. 1. Section along the middle axis of the SAXS camera, perpendicular to the plane of the primary beam. The vertical scale is multi fold stretched compared to the horizontal one.
constructed to maximize the intensity on the sample using a slit collimation system. The performance of this camera can be further increased if the primary beam is collimated from a divergent into a focusing beam. A recently developed device for this purpose is the so-called ‘‘Max-Fluxt Optic’’. It consists of an elliptically bent multilayer designed to collimate divergent Xrays from laboratory X-ray sources into a focused and monochromatic beam of high brilliance. Modification of the block collimation system in combination with the Max-Fluxt Optics lead to a different beam geometry resulting in an intensity increase by a factor of about 20 compared to the Kratky Compact Camera [4]. Fig. 1 shows the scheme of the setup. The camera consists of a rotating anode generator, an elliptically bent multilayer X-ray mirror and a block collimation system. The whole system can be evacuated. The multilayer’s d-spacing is graded along the length of the curved optic to satisfy the Bragg condition for every point along the optic. Elliptically bent multilayer optics allow to transform the divergent beam of an X-ray source to a focused beam. Fig. 2 shows the principles of the multilayer optics [5]. This optics in combination with a rotating anode should deliver the required brilliance to reduce the detection limits in TXRF.
The rotating anode1 with Cu target offers a filament size of 0.1 10 mm2, the maximum power used in this configuration is 4 kW (40 kV 100 mA). The multilayer optics2 shows focusing lengths of 150 and 500 mm (see Fig. 1). The optics is installed in a new high flux SAXS camera.3 The TXRF sample adjustment mechanism was designed and constructed at the Atominstitut. The sample reflector was a 30 mm quartz reflector and positioned at the sample position in Fig. 1 in the vacuum chamber. Fig. 3 shows the setup. The adjustment was performed with 2 rotation vacuum feedthroughs followed by an excentric motion, allowing the adjustment of the position of the reflector relative to the beam to excite in optimized position, as well as with a linear feed through for angle adjustment. The detector used was a Si drift chamber detector4 with an active area of 5 mm2 and resolution of 148 eV at 5.9 keV. The thickness of the Be window was 7 mm. As the area of the detector is small, the sample reflector has to be siliconized to achieve small (o1 mm) sample spots.
1
Siemens M18XCE, AXS Bruker, Karlsruhe, Germany. Osmic Inc., Troy, USA. 3 Anton-Paar GmbH, Graz, Austria. 4 KETEK, Munich, Germany. 2
P. Wobrauschek et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 569–572
571
Fig. 2. Multilayer coated X-ray optic, combining a figured optic with a multilayer, which is graded across the length of the optic [5].
Elliptically bent multilayer X-ray optics
Block collimation system
Detection plane of the scattered radiation
Sample position
A
R
SDDetector
B2 f DS k1'
k2'
H
k1
p
O2
m0
B1
X
Rotating anode
k2
O1 w
C 2
C 2
C 2
3C 2
TXRF sample reflector Fig. 3. Setup for TXRF at the high flux SAXS camera.
3. Results The first sample measured was a Merck standard, diluted from 1000 mg/l, containing 100 mg/l of Co and 3 ml pipetted on the sample reflector and dried. Fig. 4 shows the spectrum obtained. The attention should be drawn to the background, which is very low. The detection limits derived from this spectrum are 300 fg for 1000 s measuring time. Detection limits are defined as 3 times square root of background counts
divided by net counts and multiplied by the sample mass. The result of 300 fg can be compared to about 1 pg obtained with a 2 kW fine-focus standing anode tube with non-focusing multilayer monochromator [6] and to about 10 fg using monochromatic synchrotron radiation from a bending magnet beamline [7]. For a serious comparison one has to consider that the results cited were obtained with a 30 mm2 SiLi detector and the detector used in the mentioned experiment was only 5 mm2.
P. Wobrauschek et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 569–572
572
Table 1 Detection limits determined from SRM 1643d from NIST
2500 300 pg Co Cu 40 kV 100mA 100s
Si 2000
cts/chn
Co 1500
1000
Cu-scatter
500
0
Ar Ca Cl K 0
2
6
8
10
Energy (keV)
Fig. 4. Spectrum of a sample containing 300 pg Co, excited with 40 kV and 100 mA, measuring time 100 s.
NIST water SRM 1643d
4
6.4 10
K
4
Ca
Si
5.6 10
SRM NIST 1643d Water sample Cu 40 kV 100mA 100s in µg/l
4
cts/chn
4.8 10 4 10
LLD1000 (fg)
LLD1000 (ng/l)
V Cr Mn Fe Co Ba
35 18 37 72 25 506
4541 1137 1772 901 1088 7109
1514 379 591 300 363 2370
The results show that the high flux SAXS camera with rotating anode tube as excitation source for TXRF leads to improved detection limits in comparison to standing anode tube excitation with planar multilayer monochromator. This combination would be a compromise between standard laboratory excitation and excitation with synchrotron radiation.
Cu-scatter
Acknowledgements
4
4
3.2 10
Mg 22000
4
2.4 10
Ba 506
Ca-Esc
Fe 72
V 35 Cr 18
S
8000 0
2
The authors would like to thank Walter Drabek from the Atominstitut for the machining of the mechanical components and Rupert Schwarzl from the Institute of Chemistry for his help during the preparation of the experiment and the FWF (# PHY14336) for financial support.
Σ Ca
Na 8000
4
1.6 10
0
Csample (mg/l)
4. Conclusions
Fe
4
Element
Mn 37
4
6
Co 25
8
Energy(keV)
Fig. 5. Spectrum of a sample of 10 ml SRM 1643d from NIST, excited with 40 kV and 100 mA, measuring time 100 s. Concentration values from the reference list in mg/l.
The second sample was a standard reference material from NIST (1643d—water sample). Ten microlitre sample volume was pipetted and dried on the reflector. Fig. 5 shows the measured spectrum. Besides the main elements K and Ca several trace elements were found. The derived detection limits for 1000 s as well as the sample concentration from the reference list are presented in Table 1.
References [1] P. Wobrauschek, H. Aiginger, Anal. Chem. 47 (1975) 6. [2] P. Wobrauschek, C. Streli, in: R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, Wiley, New York, 2000. [3] P. Wobrauschek, C. Streli, X-ray and inner-shell processes, in: R.L. Johnson, H. Schmidt-B.oacking, B. Sonntag (Eds.), AIP Conference Proceedings, Vol. 389, AIP Press, Woodbury, NY, 1997, p. 233. [4] A. Bergmann, D. Orthaber, G. Scherf, O. Glatter, J. Appl. Cryst. 33 (2000) 869. [5] http://www.osmic.com/products/max-flux/max-flux.html. [6] W. Ladisich, R. Rieder, P. Wobrauschek, H. Aiginger, Nucl. Instr. and Meth A 330 (1993) 501. [7] R. Rieder, P. Wobrauschek, W. Ladisich, C. Streli, H. . Aiginger, S. Garbe, G. Gaul, A. Knochel, F. Lechtenberg, Nucl. Instr. and Meth. 355 (2,3) (1995) 648.
Letter to the Editor
Total Reflection X-ray Fluorescence Analysis (TXRF) using the high flux SAXS camera P. Wobrauscheka,*, C. Strelia, G. Pepponia, A. Bergmannb, O. Glatterc a
Atominstitut, TU Wien, Stadionallee 2, 1020 Vienna, Austria b Anton Paar GmbH, Graz, Austria c Institute of Chemistry, Graz, Austria Received 2 January 2002; accepted 22 January 2002
Abstract Combining the high photon flux from a rotating anode X-ray tube with an X-ray optical component to focus and monochromatize the X-ray beam is the most promising instrumentation for best detection limits in the modern XRF laboratory. This is realized by using the design of a high flux SAXS camera in combination with a 4 kW high brilliant rotating Cu anode X-ray tube with a graded elliptically bent multilayer and including a new designed module for excitation in total reflection geometry within the beam path. The system can be evacuated thus reducing absorption and scattering of air and removing the argon peak in the spectra. Another novelty is the use of a Peltier cooled drift detector with an energy resolution of 148 eV at 5.9 keV and 5 mm2 area. For Co detection limits of about 300 fg determined by a single element standard have been achieved. Testing a real sample NIST 1643d led to detection limits in the range of 300 ng/l for the medium Z. r 2002 Published by Elsevier Science B.V.
1. Introduction Total Reflection X-ray Fluorescence Analysis (TXRF) is a special technique in energy dispersive X-ray fluorescence analysis achieving detection limits in the pg range (or ppb-levels) with standing anode 2 kW fine focus X-ray tubes [1,2]. The technique is based on the total external reflection of X-rays on the smooth polished surface of a quartz or Si reflector if the beam impinges under an angle lower than the critical angle. TXRF requires therefore a low divergent beam with high intensity. Synchrotron radiation is the ideal source
for TXRF and allows in reducing the detection limits down to the fg level [3]. To improve the laboratory performances of TXRF an increase of photon flux for sample excitation is required. A high brilliant rotating anode tube with focusing optics should improve the detection limits. Experiments with the high flux SAXS (small angle X-ray scattering) Camera at the Institute of Chemistry, University of Graz, have been performed.
2. Experimental *Corresponding author. E-mail address: [email protected] (P. Wobrauschek).
Laboratory X-ray sources emit a highly divergent beam. The Kratky compact camera is
0168-9002/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 4 6 3 - 1
570
P. Wobrauschek et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 569–572
Elliptically bent multilayer X-ray A optics
Block collimation
Detection plane of the scattered radiation
Sample position
system
R
B2 f
H
w
DS
k 1'
X
Rotating anode generator
k 2'
k2
O1 k1
p
O2
m0
B1 C 2
C 2
C 2
3C 2
C = 180 mm Fig. 1. Section along the middle axis of the SAXS camera, perpendicular to the plane of the primary beam. The vertical scale is multi fold stretched compared to the horizontal one.
constructed to maximize the intensity on the sample using a slit collimation system. The performance of this camera can be further increased if the primary beam is collimated from a divergent into a focusing beam. A recently developed device for this purpose is the so-called ‘‘Max-Fluxt Optic’’. It consists of an elliptically bent multilayer designed to collimate divergent Xrays from laboratory X-ray sources into a focused and monochromatic beam of high brilliance. Modification of the block collimation system in combination with the Max-Fluxt Optics lead to a different beam geometry resulting in an intensity increase by a factor of about 20 compared to the Kratky Compact Camera [4]. Fig. 1 shows the scheme of the setup. The camera consists of a rotating anode generator, an elliptically bent multilayer X-ray mirror and a block collimation system. The whole system can be evacuated. The multilayer’s d-spacing is graded along the length of the curved optic to satisfy the Bragg condition for every point along the optic. Elliptically bent multilayer optics allow to transform the divergent beam of an X-ray source to a focused beam. Fig. 2 shows the principles of the multilayer optics [5]. This optics in combination with a rotating anode should deliver the required brilliance to reduce the detection limits in TXRF.
The rotating anode1 with Cu target offers a filament size of 0.1 10 mm2, the maximum power used in this configuration is 4 kW (40 kV 100 mA). The multilayer optics2 shows focusing lengths of 150 and 500 mm (see Fig. 1). The optics is installed in a new high flux SAXS camera.3 The TXRF sample adjustment mechanism was designed and constructed at the Atominstitut. The sample reflector was a 30 mm quartz reflector and positioned at the sample position in Fig. 1 in the vacuum chamber. Fig. 3 shows the setup. The adjustment was performed with 2 rotation vacuum feedthroughs followed by an excentric motion, allowing the adjustment of the position of the reflector relative to the beam to excite in optimized position, as well as with a linear feed through for angle adjustment. The detector used was a Si drift chamber detector4 with an active area of 5 mm2 and resolution of 148 eV at 5.9 keV. The thickness of the Be window was 7 mm. As the area of the detector is small, the sample reflector has to be siliconized to achieve small (o1 mm) sample spots.
1
Siemens M18XCE, AXS Bruker, Karlsruhe, Germany. Osmic Inc., Troy, USA. 3 Anton-Paar GmbH, Graz, Austria. 4 KETEK, Munich, Germany. 2
P. Wobrauschek et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 569–572
571
Fig. 2. Multilayer coated X-ray optic, combining a figured optic with a multilayer, which is graded across the length of the optic [5].
Elliptically bent multilayer X-ray optics
Block collimation system
Detection plane of the scattered radiation
Sample position
A
R
SDDetector
B2 f DS k1'
k2'
H
k1
p
O2
m0
B1
X
Rotating anode
k2
O1 w
C 2
C 2
C 2
3C 2
TXRF sample reflector Fig. 3. Setup for TXRF at the high flux SAXS camera.
3. Results The first sample measured was a Merck standard, diluted from 1000 mg/l, containing 100 mg/l of Co and 3 ml pipetted on the sample reflector and dried. Fig. 4 shows the spectrum obtained. The attention should be drawn to the background, which is very low. The detection limits derived from this spectrum are 300 fg for 1000 s measuring time. Detection limits are defined as 3 times square root of background counts
divided by net counts and multiplied by the sample mass. The result of 300 fg can be compared to about 1 pg obtained with a 2 kW fine-focus standing anode tube with non-focusing multilayer monochromator [6] and to about 10 fg using monochromatic synchrotron radiation from a bending magnet beamline [7]. For a serious comparison one has to consider that the results cited were obtained with a 30 mm2 SiLi detector and the detector used in the mentioned experiment was only 5 mm2.
P. Wobrauschek et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 569–572
572
Table 1 Detection limits determined from SRM 1643d from NIST
2500 300 pg Co Cu 40 kV 100mA 100s
Si 2000
cts/chn
Co 1500
1000
Cu-scatter
500
0
Ar Ca Cl K 0
2
6
8
10
Energy (keV)
Fig. 4. Spectrum of a sample containing 300 pg Co, excited with 40 kV and 100 mA, measuring time 100 s.
NIST water SRM 1643d
4
6.4 10
K
4
Ca
Si
5.6 10
SRM NIST 1643d Water sample Cu 40 kV 100mA 100s in µg/l
4
cts/chn
4.8 10 4 10
LLD1000 (fg)
LLD1000 (ng/l)
V Cr Mn Fe Co Ba
35 18 37 72 25 506
4541 1137 1772 901 1088 7109
1514 379 591 300 363 2370
The results show that the high flux SAXS camera with rotating anode tube as excitation source for TXRF leads to improved detection limits in comparison to standing anode tube excitation with planar multilayer monochromator. This combination would be a compromise between standard laboratory excitation and excitation with synchrotron radiation.
Cu-scatter
Acknowledgements
4
4
3.2 10
Mg 22000
4
2.4 10
Ba 506
Ca-Esc
Fe 72
V 35 Cr 18
S
8000 0
2
The authors would like to thank Walter Drabek from the Atominstitut for the machining of the mechanical components and Rupert Schwarzl from the Institute of Chemistry for his help during the preparation of the experiment and the FWF (# PHY14336) for financial support.
Σ Ca
Na 8000
4
1.6 10
0
Csample (mg/l)
4. Conclusions
Fe
4
Element
Mn 37
4
6
Co 25
8
Energy(keV)
Fig. 5. Spectrum of a sample of 10 ml SRM 1643d from NIST, excited with 40 kV and 100 mA, measuring time 100 s. Concentration values from the reference list in mg/l.
The second sample was a standard reference material from NIST (1643d—water sample). Ten microlitre sample volume was pipetted and dried on the reflector. Fig. 5 shows the measured spectrum. Besides the main elements K and Ca several trace elements were found. The derived detection limits for 1000 s as well as the sample concentration from the reference list are presented in Table 1.
References [1] P. Wobrauschek, H. Aiginger, Anal. Chem. 47 (1975) 6. [2] P. Wobrauschek, C. Streli, in: R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, Wiley, New York, 2000. [3] P. Wobrauschek, C. Streli, X-ray and inner-shell processes, in: R.L. Johnson, H. Schmidt-B.oacking, B. Sonntag (Eds.), AIP Conference Proceedings, Vol. 389, AIP Press, Woodbury, NY, 1997, p. 233. [4] A. Bergmann, D. Orthaber, G. Scherf, O. Glatter, J. Appl. Cryst. 33 (2000) 869. [5] http://www.osmic.com/products/max-flux/max-flux.html. [6] W. Ladisich, R. Rieder, P. Wobrauschek, H. Aiginger, Nucl. Instr. and Meth A 330 (1993) 501. [7] R. Rieder, P. Wobrauschek, W. Ladisich, C. Streli, H. . Aiginger, S. Garbe, G. Gaul, A. Knochel, F. Lechtenberg, Nucl. Instr. and Meth. 355 (2,3) (1995) 648.